October 22, 2005

Geographic and genetic distance in human populations

As per my previous post, a new study has shown how genetic distance is correlated with geographic distance in human populations. This is similar to another recent study on the same topic. Of interest is the observation that inter-population differences are explained mostly by genetic drift, and less by selection.

Also of interest is the greater genetic differentiation of the Mbuti pygmies and San, above that predicted by geographic distance. The authors attribute this to long-term isolation. This would seem to support my theory about the long-standing differentiation between Paleoafricans and Afrasians. The San and the Mbuti are significantly derived from the Paleoafrican groups who were reproductively isolated from the Afrasians who were the most recent wave of modern humans to spread from Africa into Eurasia and the interior of Africa itself.

PNAS (Early view)

Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa Sohini Ramachandran et al.

Equilibrium models of isolation by distance predict an increase in genetic differentiation with geographic distance. Here we find a linear relationship between genetic and geographic distance in a worldwide sample of human populations, with major deviations from the fitted line explicable by admixture or extreme isolation. A close relationship is shown to exist between the correlation of geographic distance and genetic differentiation (as measured by FST) and the geographic pattern of heterozygosity across populations. Considering a worldwide set of geographic locations as possible sources of the human expansion, we find that heterozygosities in the globally distributed populations of the data set are best explained by an expansion originating in Africa and that no geographic origin outside of Africa accounts as well for the observed patterns of genetic diversity. Although the relationship between FST and geographic distance has been interpreted in the past as the result of an equilibrium model of drift and dispersal, simulation shows that the geographic pattern of heterozygosities in this data set is consistent with a model of a serial founder effect starting at a single origin. Given this serial-founder scenario, the relationship between genetic and geographic distance allows us to derive bounds for the effects of drift and natural selection on human genetic variation.

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